The most important function to monitor is oxygen supply to the brain, e.g. for example during anaesthesia. The measurement of this is known as pulse oximetry. The brain can only survive on oxygen and glucose, i.e. by aerobic metabolism, whereas most other tissues can survive for a period without oxygen, is by anaerobic metabolism. The oxygen supply is a function of delivery, i.e. cardiac output which governs all blood flow, and the amount of oxygen available in the blood, which is governed by the O.sub.2 haemoglobin dissociation curve.
The blood flow is assessed by monitoring blood pressure and by assuming the resistance is constant, so the pressure is directly related to flow (Ohm's law) and by pulse rate which is generally measured with an electro-cardiogram (ECG) and/or a pulse oximeter. The amount of oxygen available is measured with a pulse oximeter which calculates the percentage saturation of the haemoglobin by using one, two or three mono-chromatic light sources. The amount of light passing through tissue of the body is recorded to determine the quantity of light absorbed by the haemoglobin. These devices usually measure the saturation in the periphery, e.g. in the toe, finger or ear, and this is assumed to be related to the oxygen saturation of blood delivered centrally to the brain.
The major limitation of pulse oximetry is that the measures are of peripheral and not central blood flow. Other limitations of pulse oximetry are;
(i) The calculations are based on the Beer Lambert law and it is known that due to the scattering of light by skin tissue and other structures the application of this law is not valid under these conditions.
(ii) The wavelengths of light used are usually determined by the available light emitting diodes and these may not be ideal. Existing machines use only one, two or three wavelengths which further limits the accuracy of the oxygen saturation calculation.
(iii) Ambient light may interfere with the signal detection and hence reduce its sensitivity.
(iv) The spectral absorbance of the tissue through which the light is transmitted affects the results, e.g. with non-Caucasians.
(v) Other chemicals normally found in the blood, e.g. bilirubin, may also interfere with the transmission of the light and have an unknown influence on the results.
The electro-retinogram (ERG) is also used to assess the function of the retina. This device uses a contact lens provided with a circular electrode for detecting electrical activity of the retina. Stimulation is provided by an external light source directed into the eye.
Contact lenses provided with mirrors or lenses have also been used for observing the angle of the anterior chamber and the peripheral fundus of the eye. A high-negative-powered contact lens has also been used to assist observation of the retinal fundus during opthalmoscopy and a telescopic device has been used on a contact lens for the study of stabilised retinal images.
The size of the pupil may be used as a clinical guide during anaesthesia and infra-red pupillometry has been described as a method of assessing various aspects of visual functions. In this method, an external infra-red camera is used to measure the size of the pupil without influencing its size. The use of a train of four electrical pulses to stimulate a muscle to assess neuro-muscular blockage during anaesthesia has also been described.
Visual evoked potentials (VEP), have also been used to monitor the condition of a patient during anaesthesia and at other times. The potentials are measured by recording changes in the electrical activity of the brain following a measured visual stimulus or series of measured visual stimuli. An electro-cardiogram (ECG) is used to measure the electrical activity of the heart and as such provides a measure of heart-rate, heart rhythm, muscular contraction and, indirectly, oxygen supply to the muscle.
One of the aims of the present invention is to provide a device for non-invasive, real time monitoring of cardiovascular, respiratory, neuro-muscular and other bodily functions whilst avoiding some of the disadvantages of the prior art.